The manufacture of semiconductor circuits is primarily accomplished with photolithographic techniques. During this manufacturing process, multiple layers of a circuit pattern are built up on a semiconductor wafer. This is accomplished by projecting an image of a mask containing the circuit pattern onto a wafer coated with a photosensitive compound known generally as photoresist. The feature sizes of the circuit patterns formed on the semiconductor wafer are typically in the range of 0.50 microns or smaller. Due to these extremely small feature sizes and the requirement to expose multiple layers as part of the process in forming the semiconductor chip, the use of an alignment system to align the mask image to the semiconductor wafer to an accuracy better than 0.1 micron is required.
One such alignment system is disclosed in U.S. Pat. No. 4,697,087 entitled "Reverse Dark Field Alignment System for Scanning Lithographic Aligner" issued to Frederick Y. Wu on Sept. 29, 1987, which is herein incorporated by reference. Therein disclosed is an alignment system wherein, a wafer having a wafer target thereon, and a mask having a mask target thereon, are aligned with respect to each other. The alignment system has two optical channels, or arms, used to detect alignment targets in scribe alleys above and below the mask pattern being imaged on the semiconductor wafer. A portion of the light path used in the two optical channels, or arms, is through the projection optics used to image the mask containing the circuit patterns thereon onto the wafer. This system is referred to as an off-axis through-the-lens alignment system. While the off-axis through-the-lens alignment system has several advantages, its nature and inherent complexity lead to several deficiencies that are outlined below.
During an overlay alignment sequence the photosensitive resist coating wafer cannot be exposed. The deep ultraviolet, or UV, actinic wavelengths used to expose the photosensitive resist coating therefore cannot be used by the off-axis through-the-lens alignment system. The off-axis through-the-lens alignment system uses non-actinic, visible wavelength, narrow band laser light. Because the projection optics are designed to perform optimally at the photolithographic actinic wavelengths (i.e. the deep UV), the projection optics' performance at the off-axis through-the-lens alignment system's visible wavelengths is somewhat compromised. The alignment system therefore requires corrections due to color aberrations of the projection optics at the alignment system's visible wavelengths compared to the actinic UV wavelengths. These adjustments for color aberrations must be periodically recalibrated at the different optical alignment channel or arm positions.
Additionally, the off-axis through-the-lens alignment system's use of narrow band laser light results in a wide variation in alignment signal strength and shape depending on the wafer surface and coating characteristics. Wafer characteristics, such as the number, thickness, and type of surface layers, vary depending upon the process required to achieve the final product. The variation in alignment signal as a function of these process variations is referred to as process sensitivity. Process sensitivity for the off-axis through-the-lens alignment system is exacerbated by the use of laser light because of the potential for interference effects caused by the layer structure covering the wafer alignment marks.
Therefore, there is a need for an alignment system that can operate independent of the projection optics. This allows both the projection optics and the alignment system to be optimized independently with no compromise between the two systems. In particular, this separation enables the alignment system to be designed such that no calibration or adjustment is needed for color correction as in the through-the-lens alignment system. It allows the alignment system to be designed to work with broadband light to eliminate the interference effects and thus reduce process sensitivity. Additionally, the need for a simpler less expensive alignment system that is accurate, reliable and easily maintained can be more easily achieved by bypassing the projection optics. The use of projection optics as part of the alignment system increases the cost and complexity, as well as the required maintenance while potentially reducing the reliability of the photolithographic scanner.